Nick C
The battery has nothing to do with FPS. Only ROF changes with batteries. The only thing you can do to bring down FPS is use heavier BBs or get a new spring.

meisignedin
The battery you use has no effect on FPS, it just determines how fast it cycles and how much it can pull on the spring. If you want to reduce the FPS, you'll need to downgrade the spring.

Anonymous M
The battery only alters the ROF not the FPS (The lesser the voltage the lower the ROF, The higher the voltage the higher the ROF) (Oh and by the way Li-Po batteries give a much higher rate of fire than a Nicad battery of the same voltage) also the size of the battery will not affect ROF but battery live if your wondering. The only way to lower the FPS is to buy a softer spring and with a softer spring I recommend a softer hop-up bucking because a hard bucking is too hard for a gun with a lower FPS than 400 with a .20 gram bb.